The present invention relates to heat storage systems, particularly systems for storing solar energy. Other applications include the conservation of heat energy by recycling discharged heat.
Several heat storage systems are available which store heat in the form of the latent heat of fusion. An example of such a system is found in U.S. Pat. No. 2,677,243 to Telkes. In such a system, a heat transfer fluid is passed over containers which contain special heat storage substances. These substances are selected to have a melting point less than the prime heat source and equal to a desired output temperature. Such systems have several advantages over non-fusion type systems. For one, any substance has its greatest heat storing capacity at the melting temperature because a much greater amount of heat is required to melt a substance at its constant melting temperature than is required to raise the temperature of a solid or liquid several degrees. Hence, when the temperature of the substance must pass through the melting temperature in order to reach a higher transfer fluid temperature a large amount of heat will be absorbed and thus stored as latent heat of fusion.
Another advantage of fusion type systems is that the bulk of the heat stored in the substance will be retrieved at the constant melting temperature. Hence, as was suggested by Telkes, a storage substance can be selected such that its melting temperature is at the desired output temperature.
One disadvantage of prior systems is that, with time, the storage substance inevitably breaks down so that subsequently relatively little heat can be stored therein. Prior storage units were simply disposed of after breakdown or sent back to the manufacturer for costly refurbishing.
Another disadvantage of prior systems was the lack of flexibility. In past systems, with changes in heat source or heat requirements a heat storage system could be rendered useless, requiring complete replacement. Nor could a system be controlled to provide one temperature output over one period of time and another temperature output over another period of time.
Still another disadvantage of prior art systems is that the heat transfer fluid must have a temperature greater than the melting temperature of the storage substance in order to store a substantial amount of heat. Hence, if the source temperature varied, it might be necessary to store at a lower temperature than might be desirable in order to absorb heat over a longer period within the varying heat cycle. In order to store more heat at this lower temperature level, larger units were required. An example of this problem can be found in a solar heating system wherein the heat source might vary from unusable temperatures at night to 300.degree. F. or higher during the day. If a storage medium having a melting temperature at 250.degree. F. were used, the storage system could be used as a high temperature output source, but because of the short period during which fluid at a temperature greater than 250.degree. F. would be available, exceptionally large and efficient solar collectors would be required in order to supply a sufficient amount of heat to activate the storage system. On the other hand, if the melting temperature of the storage medium were set at 100.degree., heat could be transferred into the storage system over a greater period; however, the temperature might not be sufficient for such output uses as home heating and laundry.